I am wondering on the gearing I should be running? I am not sure what I have in the rear now, but it is a stock 8.2gm 10 bolt 1968, but my tach says I am around 2800-2900 rpm @ 60-65mph I also have a team g single plane intake with a 670 holley vac secondaries on there, could that single plane intake be the cause of sluggish bottom end? But once it gets the RPM up it flies like a bat out of hell! Also I have 245 50 r 15 on the front and 275 60r 15 on the rear which I think calculates to 31" tall tires for the rear. Pls help I am happy with mid to top end performance but would like recommendation on gears and posi? Some have told me posi is to dangerous easy to loose control, breaking loose all the time from the road. I will be driving a good amount of highway driving and not a strip car. I will be sticking with this th350 for the time being, but next time a OD trans for sure. Sees like with my RPM at 65 could it be 3.73's? If so how can it be so sluggish down low if in fact it is 3.73 gears, dual plane intake???

The 265/60 tire is 28" tall. At 2900rpm w/3.73 I get 65mph, w/4.10 I get 59mph.
A Dual Plane Intake such as Edelbrock RPM would work better with the Cam you have. Slightly more Stall @2500rpm would also but, not neccesary.
What is your Ignition timing advance set at, Initial, Mechanical and Vacuum advance?
Timing Info:http://www.corvette-restoration.com/.../Timing101.pdf

A Posi Diff can be a little touchy on wet surfaces but, dry surfaces it's not a problem.

A 2004R O/D Trans would be a good swap, same length as the TH350, better gear ratios. Bolts right in using a TH400 Crossmember. Requires a TV Cable & Brackets and a simple Lockup Kit for the TCC/TorqueConverterClutch.